This invention relates to a method of fabricating solid state integrated circuits and, in particular, to a method of encapsulating selected areas of the circuits.
As is well known in the art of micorelectronic circuit fabrication, an important step in the processing is to protect the components from the atmosphere by applying a suitable layer of encapsulating material. This material is often a silicone rubber dissolved in an organic solvent, and is formed over the entire circuit by a flow coating technique. In many instances in the production of thick film, thin film, and hybrid integrated circuits, it is desirable to leave certain portions of the circuit unencapsulated. For example, it is often desirable to test portions of the circuit after the encapsulant is applied, since the encapsulant will have some effect on the capacitances in the circuit. For the same reason, it is also desirable to leave certain resistors or capacitors unencapsulated so they can be trimmed subsequent to encapsulation of other elements in the circuit. In addition, it may be preferable in the manufacture of bi-level circuits to prevent the encapsulant from running into the via holes which are utilized for electrical connection between elements on both sides of the substrate.
At the present time, selective encapsulation can be performed by employing special fixtures such as rubber masks which are clamped down on the substrate, or by constructing dams to prevent the flow of encapsulant to the areas to be unencapsulated. The former method requires special fixtures for each substrate and can cause damage to circuit components, and the latter provides limited resolution, especially when used with flow coating encapsulation.
It is, therefore, a primary object of the invention to provide a method for selective encapsulation which does not require special fixtures and which provides good resolution with flow coating encapsulations.